The present invention relates to methods and apparatus for inducing a varying second-order non-linearity profile in optical fibres and other waveguides, and to waveguides having such a varying second-order non-linearity profile.
Waveguides with periodically varying second-order non-linearities are of interest, since they can be used to provide quasi-phase matching (QPM). In non-linear glasses, QPM is usually achieved by periodically alternating regions with non-linearity (poled sections) and regions without non-linearity (unpoled sections). In this way the phase-mismatch accumulated in each poled section is reset in the unpoled one (where the absence of the non-linearity prevents back conversion), so that in the next poled section the fields have the right phase relation for constructive growth of the generated signals.
QPM devices have potential applications for optically integrated frequency conversion of coherent light sources, wavelength routing in telecommunication systems, all-optical switching via cascading of second-order non-linearities, parametric fluorescence for quantum applications (such as cryptography and metrology), and high speed modulation.
Since its proposal [1] QPM has been implemented in many materials including lithium niobate, semiconductors and polymers. Several configurations have been employed to achieve efficient second-order non-linear optical interactions. QPM allows one to access new wavelengths and to provide higher efficiency and non-critical interaction geometries. QPM also provides flexibility and new possibilities for phase-matching, especially in materials where the birefringence is not high enough to compensate for the dispersion and where modal phase-matching is not desirable in order to avoid the generation of light in higher order modes.
QPM devices can be fabricated by periodic poling of waveguides. Here and throughout this document the term xe2x80x9cperiodic polingxe2x80x9d is used to mean a varying second-order non-linearity profile, not necessarily of a single frequency. Periodic poling exploits the potential of the QPM technology to extend the possibility of efficient frequency conversion to materials, which are in widespread use in optical applications, such as silica and germanosilicate optical glass. This is advantageous since silica and some other optical glasses exhibit high transparency, are low cost, have high optical damage thresholds, and are straightforward to integrate with optical fibre and planar waveguide-based systems.
Considering all the aforesaid properties, it is natural to consider periodically poled silica fibre (PPSF) and periodically poled silica waveguides (PPSW) as ideal media for a wide range of QPM processes, such as frequency conversion of fibre lasers, difference frequency generation for routing at telecommunication wavelengths, generation of correlated photon pairs via parametric fluorescence for quantum cryptography and avalanche photodiodes characterisation, and cascading of second-order non-linearities to produce equivalent third order effects (self and cross-phase modulation) for all-optical switching. In addition to the above applications which are based on three-wave-mixing (TWM) processes, periodic modulation of a second order non-linearity (hence the electro-optic coefficient) could be exploited to produce high speed travelling wave electro-optic switches.
Compared to more traditional crystal waveguides, PPSF has the drawback of a lower effective non-linear coefficient (deff), but offers the advantages of: (i) a longer interaction length (L) for the same bandwidth (due to a lower dispersion); (ii) higher damage intensity threshold (I); and (iii) lower loss (xcex1) and refractive index (n), thus keeping high values for the efficiency-factor (deff2L2I)/(xcex1n3). In particular, the large value of the bandwidth-interaction length product makes PPSF suitable for frequency conversion of short pulses (picosecond and even femtosecond) where low group velocity mismatch between interacting pulses at different frequencies is desirable.
Production of a permanent and large second-order non-linearity in fused silica glass was demonstrated some time ago [2]. However, later initial work on QPM in optical fibres [3] relied on a different process which produced a non-permanent second-order non-linearity. Later work on QPM [4] is based on a permanent and large second-order non-linearity induced in fused silica by a combined thermal and electrical process in which a high voltage is applied between electrodes across a waveguide while the waveguide is maintained at a relatively high temperature. This process which involves elevated temperature is referred to, at least in the present document, as thermal poling. By structuring one of the electrodes, the thermal poling can be selectively induced only in those regions of the waveguide underlying the structured electrode. In this way a varying profile of the second order non-linearity can be induced. This is referred to as periodic thermal poling (PTP), at least in the present document, where it shall be understood that the term xe2x80x9cperiodicxe2x80x9d does not imply that the second order non-linearity profile is necessarily of a single frequency component. More complex profiles (e.g. chirped) are also to be understood to be encompassed by this term.
PTP has been demonstrated in silicate glass bulk [5] and in optical fibre [6] to produce permanently poled structures. The use of planar lithography on a D-shaped fibre to define a patterned electrode for PTP has also been demonstrated [7] and subsequently used for highly-efficient frequency-doubling of femtosecond pulses [8], and for high power fibre sources [9]. A technique for planar lithography has been described [10] and the use of such technique to define periodic structure was suggested, however not supported by any experimental data.
Another method to produce a varying second order non-linearity profile, which is referred to, at least in the present document, as periodic optical poling (POP), is based on ultra-violet (UV) light exposure. It shall be understood that the term xe2x80x9cperiodicxe2x80x9d does not imply that the second order non-linearity profile is necessarily of a single frequency component. More complex profiles are also to be understood to be encompassed by this term. POP methods have been previously described [11], [12] and [13]. In reference [11] POP is performed by thermal poling and subsequent UV erasure using an interference pattern. In references [12] and [13], POP is performed by the simultaneous application of an electric field and exposure to a periodic UV pattern.
One of the advantages of POP over PTP is the possibility of fabricating longer samples. Existing PTP methods rely on direct lithography of a patterned electrode. For single-step lithography, sample length is therefore limited to about 10 cm.
In POP a combination of uniform thermal poling and selective UV erasure can be used. (Here the term uniform thermal poling is used to distinguish from PTP to express the induction of a continuous, substantially constant, second order non-linearity along a waveguide). Neither uniform thermal poling or periodic UV erasure imposes a short length requirement. Uniform thermal poling does not require any electrode patterning, so no masks are needed. Periodic UV erasure can be performed on lengths of up to 1 m with periods of down to about 0.5 xcexcm using techniques developed for fabricating fibre Bragg gratings, i.e. inducing refractive index modulation in photosensitive optical fibre.
It is important to note that for POP it is not necessary to pattern an amplitude mask directly onto the sample, so that the photolithography step is not needed. Therefore another advantage lies in the possibility to use a fibre possessing a geometry closer to standard telecommunication ones (outer diameter xcx9c125 xcexcm), on which the realisation of a periodic electrode by means of standard lithography would be fairly awkward, in particular for short periods.
The typical QPM periods for forward three-wave mixing (i.e. the three interacting waves are propagating in the same direction) in glass-based optical fibres and planar waveguides are between 10 and 100 xcexcm. For backward interaction (one of the waves is propagating in the opposite direction with respect to the other two waves) periods in the range of 0.3-1 xcexcm are desirable. These shorter periods are readilly achievable with POP. With PTP, sub-micron lithographic processing would need to be be employed.
Another interesting feature of glass is the possibility of combining linear refractive index and second-order non-linear gratings in the same fibre or planar waveguide device. This offers the possibility of making, for example, a fibre-based fully-integrated optical parametric oscillator where mirror feed-back is provided by a linear Bragg grating and a second-order non-linear grating provides a gain medium. The combination of linear and non-linear grating could also lead to enhanced efficiency [14, 15].
According to one aspect of the invention there is provided a method of creating a second order non-linearity profile along a waveguide, comprising: thermally poling a waveguide structure to generate a second order non-linearity; placing a mask adjacent to the waveguide structure; and exposing the waveguide structure with UV light through the mask to selectively erase the second order non-linearity along the waveguide structure. The mask may be an amplitude mask or a phase mask.
According to another aspect of the invention there is provided an apparatus for selectively erasing thermal poling, comprising: a sample holder for mounting a thermally poled waveguide structure; a mask removably held in fixed relation to the sample holder; and a UV light source arranged to illuminate a thermally poled waveguide structure mounted in the sample holder through the mask.
Further aspects of the invention relate to waveguide structures and optical devices made according to the method and apparatus of the above aspects of the invention.
In the following detailed description, waveguide geometry considerations are discussed in detail. For example, the importance of the distance between electrodes during thermal poling is discussed in the light of the mechanism of depletion region formation and the measurements on fused silica bulk samples.
It is also described how a large fibre diameter and/or a fibre with one or more internal holes can be advantageously used to avoid electrical breakdown during thermal poling and provide the possibility for thermal poling in air and other non-vacuum conditions.
Furthermore, it is described how POP can be used for fabricating longer non-linear gratings using amplitude masks and the realisation of shorter periods using phase masks. Short periods are suitable for backward interactions and for highly dispersive materials.
The methods described also allow the realisation of complex combinations of linear and non-linear structures.
The varying second-order non-linearity profiles realisable with embodiments of the invention can be used to fabricate various devices. In the case of three wave mixing processes, the profile allows compensation of the phase-mismatch between the interacting waves and the corresponding non-linear driving polarisation (i.e. QPM). A waveguide structure and poling conditions are used which maximise the overlap between the interacting waves and the poled region, the efficiency of the interaction being proportional to the degree of overlap.
In an illustrative embodiment of the invention periodically poled D-shaped germanosilicate fibres are fabricated using uniform thermal poling and a photolithographically defined electrode that is used as an amplitude mask for subsequent selective UV erasure. The use of uniform pre-etching is also discussed for preparing the sample prior to deposition of the photolithographically defined electrode. The results of three wave mixing experiments (including frequency doubling) carried out using the fabricated samples are then presented.
Other novel aspects and advantages of the present invention may become apparent upon reading the following illustrative description and upon reference to the drawings.